The following patents appear to be relevant prior art:
KindPat. No.CodeIssue DatePatenteeU.S. Pat. No. 6,448,924B12002 Sep. 10HaferU.S. Pat. No. 5,929,431A1999 Jul. 27HadleyU.S. Pat. No. 5,671,051A1997 Sep. 23WrightU.S. Pat. No. 5,249,470A1993 Oct. 5Hadley and CheesemanEP0089228B11991 Apr. 9Moir and ClearwaterU.S. Pat. No. 4,812,643A1989 Mar. 14TalbotU.S. Pat. No. 4,766,323A1988 Aug. 23Franklin et al.U.S. Pat. No. 4,531,408A1985 Jul. 30Chadwick and JohnsonU.S. Pat. No. 4,465,367A1984 Aug. 14SabatierU.S. Pat. No. 3,802,273A1974 Apr. 9Helmuth and ChadwickU.S. Pat. No. 3,515,485A1970 Jun. 2FrankU.S. Pat. No. 3,386,031A1968 May 28Able et al.U.S. Pat. No. 2,964,849A1960 Dec. 20RoccatiU.S. Pat. No. 2,552,739A1951 May 15Roberts
Rotor Track and Balance (RTB) is an important maintenance procedure recommended for all helicopters. Due to manufacturing tolerances and other variations between blades, certain maintenance adjustments should be made periodically in order to reduce track split and to properly balance the helicopter rotor system, correcting for mass distribution variations.
One common component of most RTB systems is the tracker, which is used to detect blade track height so that maintainers can correct track height and perform balancing. Tracker systems are well known in the prior art; various methods have been in use for decades, with techniques ranging from touching the blades with a marker, to using much more advanced optical systems. Although many different technologies exist in the prior art for helicopter rotor blade tracking, optical systems will remain the focus of this discussion.
Many optical systems use the principle of a conical view window to calculate blade track height: due to the difference in height between blades, blades following a higher flight path take longer to traverse an angular region defined by the field(s) of view of the system, and thus it remains in view for a longer period of time. By analyzing the times over which each blade is within the angular region, the height of the blade can be calculated and the proper maintenance adjustment can be recommended in order to correct the track split.
There are several systems currently in use for performing track calculation; however, they are plagued by several recurring issues that often lead to frustration and expense on the part of the maintainers and operators. One such issue is environmental sensitivity: most systems are passive, utilizing scattered background radiation from the sky by detecting the intensity drop as each blade passes through the field of view of the system. The use of a passive system is intuitive to many in the field due to the brightness of the background radiation; however, inherent to these designs is a reliance on environmental conditions in order for the system to function properly. In poor weather conditions, these systems are reported to experience poor functionality, often forcing maintainers to resort to strange workarounds, such as attempting to operate the trackers in night mode during the day.
Systems that include an illuminator to emit radiation towards the blades typically utilize the illuminator for night mode operation only, as the systems are unable to differentiate between the background radiation and emitted radiation. In addition, the use of a retroreflective material attached to the helicopter blades is often required; the installation and removal of these materials is not only an annoyance, but increases maintenance costs and downtime of the helicopter.